1. Field of the Invention
This invention relates to an apparatus for controlling an ignition coil of an internal combustion engine mounted on an automobile.
2. Description of the Prior Art
It is known in the ignition system of internal combustion engines to connect the primary winding of an ignition coil for generating a spark voltage in series with a power transistor and a battery and, to excite the ignition coil during the period when the power transistor is conductive and to generate a spark voltage from the secondary winding of the ignition coil at the transition of the power transistor from conduction to nonconduction. In such a kind of system, also in the transistion of the power transistor from nonconduction to conduction, a high voltage is generated in the secondary winding of the ignition coil, because the ignition coil is excited instantly. This high voltage is not required to ignite a mixture supplied to the internal combustion engine but it gives rise to disadvantages. Because, for example, in the case of a four-cylinder internal combustion engine wherein two ignition coils are employed and two ignition plugs are connected to the secondary winding of each ignition coil, the excitation of the ignition coil is started during the intake stroke and during the compression stroke of the internal combustion engine. A serious problem arises in that when a high voltage of 2,000-3,000 volts is generated in the secondary winding of the ignition coil upon excitation thereof, the ignition plugs will be ignited at the 2,000 to 3,000 volts because the air pressure inside the cylinder is close to the atmosphere at the beginning of the intake stroke and the compression stroke, and the engine operation will be unstable.
Furthermore, as the high voltage is applied to the collector of the power transistor connected in series with the primary winding of the ignition coil at the time when an electric current in the primary coil for generating a normal spark voltage is blocked, the power transistor may be destroyed.